Emitter coupled logic current reference source

ABSTRACT

A highly stable, self-starting current reference circuit for providing an accurate load current to an output load over a wide range of temperature and voltage values in which a PNP Transistor having relatively poor gain and frequency response can be coupled to a resistor network and used in conjunction with both negative and positive feedback loops to set the output load current through a reference resistor proportional to the output load current of the circuit and substantially equal to the current through the resistor network.

United States Patent 1191 Gersbach EMI'ITER COUPLED LOGIC CURRENT REFERENCE SOURCE Inventor: John Edwin Gersbach, Burlington,

International Business Machines Corporation, Armonk, NY.

Filed: Feb. 11, 1974 Appl. No.: 441,525

Assignee:

US. Cl. 323/4; 307/255; 307/297;

, 323/9 Int. CI. H03K 17/00; GOSF l/56 Field of Search 323/1, 4, 9, 22 T References Cited UNITED STATES PATENTS ll/197l Prunty 307/255 8/1972 Hoffman, Jr. et al 323/4 X SUPPLY VOLTAGE SOURCE 1451 Sept. 9, 1975 OTHER PUBLICATIONS Two-Terminal Constant-Current Device, By P. C. Allen EEE-Oct. 1965, Vol. 13, No. 10, pp. 71, 72.

Primary Examiner-Gerald Goldberg Attorney, Agent, or FirmFrancis J. Thornton [5 7] ABSTRACT A highly stable, self-starting current reference circuit for providing an accurate load current to an output load over a wide range of temperature and voltage values in which a PNP Transistor having relatively poor gain and frequency response can be coupled to a resistor network and used in conjunction with both negative and positive feedback loops to set the output load current through a reference resistor proportional to the output load current of the circuit and substantially equal to the current through the resistor network.-

4 Claims, 1 Drawing Figure LOO'IC CIRCUII PATENIEB SEP 9 75 IIIIIIIE SOURCE I/LOGII) CIRCUIT" r I I 52 I F 55 III? I SIGNAL I SOURCE I I I LOGIC CIRCUIT I L 0UT T EMITTER COUPLED LOGIC CURRENT REFERENCE SOURCE BACKGROUND OF THE INVENTION DESCRIPTION OF THE PRIOR ART There are many different circuits for current reference sources which are stable over wide ranges of transistor values, temperatures, or voltage fluctuations.

Typical of the prior art, is the U.S. Pat. No. 3,408,557 which discloses a regulating circuit providing a shunt circuit path which is so controlled that the total current from the source remains constant regardless of variations in the source, voltage, and load parameters.

U.S. Pat. No. 3,431,505 shows an Emitter Follower Circuit having a substantially constant current emitter supply which is contacted in the emitter circuit of the Emitter Follower Transistor to provide linear charge of the load capacitance upon switching.

Typical constant current generators are also shown in the IBM Technical Disclosure Bulletin Vol. 12, No. l 1, April 1970, on page 2,031. This reference discloses a constant current, a source generator, designed as a control amplifier supplying a plurality of additional current sources in which the control current is the emitter current of a transistor operating in a diode mode.

The IBM Technical Disclosure Bulletin, Vol. 15, No. 8, January 1973, on page 2,357 also teaches a controlled DC. current circuit insensitive to both voltage and temperature variations in which a pair of transistors and resistors are arranged in a network such that a fixed current flow in one of the transistors is established.

The IBM Technical Disclosure Bulletin Vol. 1 I, No. 4, September 1968, on page 421 and 422 also discloses a shunt voltage regulator that maintains a preset voltage between an output terminal and ground such that the circuit is adaptable to high or low current applications.

SUMMARY OF THE INVENTION The instant invention teaches a highly stable, selfstarting current reference circuit which willprovide an accurate load current to an output load over a wide range of temperature and voltage values. Utilization of both negative and positive feedback loops to an input transistor and a reference transistor coupled to a resistor network and a reference resistor will set the output current in a reference resistor proportional to the output current of the entire circuit and substantially equal to the current through the resistor network itself.

As a result of this arrangement, the output of the circuit is dependent only upon the difference between the reference voltage source and ground and is independent of the power supply voltages. It is therefore, a primary objective of this invention to provide a highly stable, self-starting current reference circuit.

It is another object of the invention to provide a circuit in which the output voltage level at the emitter of an emitter follower transistor in the output load circuit is held constant and independent of the reference voltage.

It is still further another object of the invention to provide'a circuit whose output is dependent on the difference between the reference voltage source and ground and independent on the supply voltage source.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of the circuit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown in schematic form an embodiment of the circuit of the invention. This circuit comprises a reference voltage source 10 coupled to a resistor network. This network is comprised of parallel resistors 11 and 12 and a diode coupled in series with resistor 12. This resistor network is in turn coupled to the emitter of a PNP transistor 14 and to the collector of an NPN reference transistor 22. The collector of transistor 14 is coupled to the base of an NPN transistor 15. The base of the PNP transistor 14 is coupled to the base of a second PNP transistor 16 and to a resistor 17. The resistor 17 is in turn connected to ground and through a resistor 18 to the emitter of transistor 16. The base of transistor 14 is also directely coupled to the collector of transistor 16 and the collector of transistor 15.

The base of transistor 15 in addition to being coupled to the collector of transistor 14 is also coupled, through a pair of serial diodes 19 and 20, in series with a resistor 21, to the emitter of a transistor 22. The emitter of transistor 15 is directly coupled to the base of the reference transistor 22 and to the base of a transistor 27 which serves as the output transistor of the current reference circuit and through a diode 23 and a series resistor 24 to a supply voltage source 25.

The emitter of reference transistor 22 is coupled through a reference resistor 26 to the supply voltage source 25. The base of transistor 22 is also coupled to the base of the circuit output transistor 27. It should be understood additional output transistors could be connected in parallel with output transistor 27 The emitter of transistor 27 is coupled through a resistor 28 to the power supply voltage source 25. The collector of output transistor 27 is coupled to a Logic Circuit which serves as a load to the current reference circuit. This logic circuit comprises a current switch consisting of a transistor 30 whose base is coupled to a souce 29 of Logic input signals, a parallel transistor 31 whose base is coupled to ground, and an emitter follower transistor 33. The emitters of both transistors 30 and 31 are coupled to the collector of the output transistor 27. The collector of transistor 30 is directly coupled to the reference voltage source 10, while the collector of transistor 31 is coupled to the same reference voltage source 10 through a resistor 32. Also directly coupled to the reference voltage source 10, is the collector of the emitter follower transistor 33 whose base is coupled to the collector of transistor 31 and whose emitter is coupled to the logic circiut output.

The above described embodiment of the invention operated satisfactorily when the resistance and voltage values were as follows:

Resistance 11 being equal to 985 ohms; resistance 12, 700 ohms; resistance 17, 2 kilohms; resistance 18, 25 ohms; resistance 21, 1.25 kilohms; resistance 24, 225 ohms, and resistances 26 and 28, both ohms.

The reference voltage souce at the reference voltage terminal being equal to +1.25 volts, while the voltage at the common return terminal or supplyv voltage source 25 being equal to 3 volts.

In the operation of the circuit of the present invention the reference voltage source and the supply reference source 25 are turned on simultaneously to apply the appropriate voltages to the circuit. Applications-of these voltages to the circuit causes, initially, a diode current flow through from the emitter of transistor 14 through its base and resistor 17, to ground. This base emitter current through transistor 14 causes transistors 14 to become conductive such that current now flows through the transistor 14 from its emitter to its collector which in turns impresses a voltage on the base of transistor causing it to also become conductive. When transistor 15 begins conduction, the base voltage applied to transistors 14 and 16 is made negative through the process of the positive feedback in the loop composed of transistors 14, 15, and 16 and transistor 16 now begins to conduct current. This positive feedbackincreases the current now flowing through transistors 15 and 16. The turning on of transistor 15 also causes transistors22 and 27 driven by the emitter of transistor 15 to become conductive. When transistor 22 turns on, most of the current through the resistor network comprised of resistors 11 and 12 and diode 13 is caused to'flow through the reference transistor 22 such that the emitter voltage of transistor 14 is reduced substantially by negative feedback action and transistor 14;conducts less. This negative feedback loop com- I pleted between the collector of transistor 22 and the emitter of transistor 14 assures that substantially all of the current of the resistor network flows through the reference resistor 26. When output transistor 27 turns on current flows through it, and through the current switch comprised of transistors 30 and 31.

If we assume that no input voltage has been applied to the transistor 30, the transistor 30 remainsinonconductive and all the current flow through the current switch flows through the resistor 32 and the transistor 31. This flow of current sets the output voltage level at the emitter of emitter follower transistor 33 at a constant value substantially independent of the reference voltage applied to the collector of the emitter follower transistor 33 by the reference voltage source 10.

When, however, a logic input signal is applied from source. 29 to the base of transistor 30, the transistor 30 becomes conductive and transistor 31 turns off. This causes the current flow through the current switch to now flow through transistor 30. When the current level through transistor 31 drops, the voltage on the base of emitter follower 33 rises to increase the voltage at the emitter of emitter follower transistor 33.

Reconsidering for a moment, transistor 14, the resistor 17 in the base leg of the transistor 14, provide a path for the base current of transistor 14 during the initial phase of conduction, thus making the circuit a selfstarting one. Similarly, resistor 18 in series with the emitter of transistor 16 reduces the sensitivity of the circuit to current gain variations in the output transis: tor 27.

By utilizing PNP transistors for transistors 14 and 16 while the remainder of the transistors in the circuit are NPNs, the current through the circuit is caused to be independent of the supply voltage source. Thus change in the voltage at the collector of the input transistor 14 does not-cause any change in current through the transistor 14.

The described circuit has the following inherent features. A combination of resistors in the resistor network composed of resistors 11 and 12, can be chosen such that the output voltage is substantially constant with variations in the supply voltage source 25, the reference supply voltage 10 and the temperature of the circuit. Further, when these resistors 11 and 12 are formed in an integrated circuit with resistor 32 they will track with resistor 32 for constant output voltage. Ideally, the resistor 18 coupled to the emitter of transistor 16 is chosen such that the reference current is increased causing the current through the transistors 22 and 27 to remain constant regardless of the changes in the current gain-of the transistors 22 and 27. Furthermore, by placing resistor 21 in series with resistor 26 instead of directly coupling the resistor 21 to the supply voltage source 25 variations in'current due to the variations in the current gain of transistor 14 are reduced. Thus, the only variations in the current through the reference resistor 26 is due to variations in the base current flowing in transistor 15, causing a current change in the output transistor 27 to be reduced substantially. Furthermore, the output current of the collector at the output transistor 27 is completely independent of the supply voltage source and variations therein. Still further, the circuit as described has highgain and the impedance at the base of output transistor 27 is low while the impedance at the collector of transistor 27 is high to minimize current output variations due to changes in component characteristics, within the feedback loops and in the load, i.e., the logic circuit.

While the invention has now been particularly shown and described with reference to a preferred embodiment', it should be understood by those skilled in the art a first NPN transistor having an emitter coupled to a voltage source, a base and a collector and means for establishing a fixed current through said NPN transistor,

said means including a second NPN transistor having a base, an emitter, and a collector, a PNP transistor having a base and a emitter-collector circuit, and means for applying a voltage to the base of said PNP transistor, said PNP transistor having its collector coupled to the base of said first NPN transistor through the base emitter circuit of said second NPN transistor and its emitter coupled to the collector of said first transistor to provide a path for negative feedback through the emitter-collector circuit of said PNP transistor and coupled through an impedance to a voltage source,

said second NPN transistor having a base connected to the collector of said PNP transistor, an emitter connected to the base of said first NPN transistor, and a collector coupled to the base of said PNP transistor to provide a positive feedback circuit through the base collector circuit of said PNP transistor and the base collector circuit of said second NPN transistor.

2. A highly stable current reference circuit for providing an accurate current to an output load over a wide range comprising an input transistor having means for coupling its emitter through an impedance to a voltage source, means for coupling its base to a first reference voltage and means for coupling its collector to the base of a reference transistor,

said reference transistor having its base coupled to an output transistor, its emitter coupled to a second reference voltage through a reference resistor and its collector coupled to said impedance and through a feedback path to the emitter of said input transistor to apply a negative feedback to said input transistor and to set the current through the reference resistor proportional to the output current of the circuit and substantially equal to the current through said impedance.

3. The reference circuit of claim 2 wherein said means for coupling the collector of said input transistor to the base of said reference transistor, includes another transistor and a positive feedback loop from the collector of said other transistor to the base of said input transistor.

4. A current reference circuit having an output load current and a current through a reference resistor proportional to the output load current of the circuit comprising a resistor network,

an input transistor, the emitter of said input transistor coupled to said resistor network,

negative feedback means coupled between the collector and emitter of said input transistor, to set the current flow through the resistor network proportional to the output load current of the circuit,

said negative feedback means including a reference transistor whose collector is coupled to the emitter of the input transistor,

a reference resistor coupled to the emitter of the reference transistor and to the collector of the input transistor and having a current flow there through proportional to the output load current of the circuit,

an output transistor, the base of said output transistor being connected to the base of said reference transistor,

positive feedback means coupled between the base and the collector of said input transistor, to compensate the output load current for variations in gain in the output transistor,

and load means coupled to the collector of said output transistor. 

1. A highly stable current reference circuit for providing an accurate current to an output load over a wide range of temperature values, transistor values or voltage fluctuations comprising a first NPN transistor having an emitter coupled to a voltage source, a base and a collector and means for establishing a fixed current through said NPN transistor, said means including a second NPN transistor having a base, an emitter, and a collector, a PNP transistor having a base and a emitter-collector circuit, and means for applying a voltage to the base of said PNP transistor, said PNP transistor having its collector coupled to the base of said first NPN transistor through the base emitter circuit of said second NPN transistor and its emitter coupled to the collector of said first transistor to provide a path for negative feedback through the emitter-collector circuit of said PNP transistor and coupled through an impedance to a voltage source, said second NPN transistor having a base connected to the collector of said PNP transistor, an emitter connected to the base of said first NPN transistor, and a collector coupled to the base of said PNP transistor to provide a positive feedback circuit through the base collector circuit of said PNP transistor and the base collector circuit of said second NPN transistor.
 2. A highly stable current reference circuit for providing an accurate current to an output load over a wide range comprising an input transistor having means for coupling its emitter through an impedance to a voltage source, means for coupling its base to a first reference voltage and means for coupling its collector to the base of a reference transistor, said reference transistor having its base coupled to an output transistor, its emitter coupled to a second reference voltage through a reference resistor and its collector coupled to said impedance and through a feedback path to the emitter of said input transistor to apply a negative feedback to said input transistor and to set the current through the reference resistor proportional to the output current of the circuit and substantially equal to the current through said impedance.
 3. The reference circuit of claim 2 wherein said means for coupling the collector of said input transistor to the base of said reference transistor, includes another transistor and a positive feedback loop from the collector of said other transistor to the base of said input transistor.
 4. A current reference circuit hAving an output load current and a current through a reference resistor proportional to the output load current of the circuit comprising a resistor network, an input transistor, the emitter of said input transistor coupled to said resistor network, negative feedback means coupled between the collector and emitter of said input transistor, to set the current flow through the resistor network proportional to the output load current of the circuit, said negative feedback means including a reference transistor whose collector is coupled to the emitter of the input transistor, a reference resistor coupled to the emitter of the reference transistor and to the collector of the input transistor and having a current flow there through proportional to the output load current of the circuit, an output transistor, the base of said output transistor being connected to the base of said reference transistor, positive feedback means coupled between the base and the collector of said input transistor, to compensate the output load current for variations in gain in the output transistor, and load means coupled to the collector of said output transistor. 